In the plastics extrusion process, raw thermoplastic material, or resin, is gravity fed from a top-mounted hopper into the barrel of an extruder. Additives, such as colorants and UV inhibitors, or other formulated compounds in either liquid or pellet form, are often used and can be introduced into the resin below arriving at the hopper. With these additives, the extruded plastic parts can meet the specific functions in different applications.
The Resin materials such as nylon, polyurethane, polyester and polycarbonate and polycarbonate are very hygroscopic, absorbing moisture rapidly from the air. At extrusion temperatures, moisture degrades these materials to lower molecular weight polymers, resulting in poorer property performance. Proper drying to eliminate moisture is critical to obtain the optimum property performance in the final product. Other materials, e.g., acrylics, Ultem polysulfone, Noryl and acrylonitrile butadiene styrene (ABS), also absorb moisture from the air and must be dried prior to processing. Any moisture in the polymer is converted to steam in the extruder and, depending on the quantity present, can cause surface imperfections such as splay, holes in the product, or a foamy product. Some polymers, e.g., nylon, are shipped dry in moisture-proof containers.
With proper handling, these resins do not normally require additional drying prior to processing. However, if the seal is broken on the container or the bag is not completely resealed after opening, the product will absorb moisture and have to be dried prior to extrusion. Polyesters are particularly sensitive to moisture and must be dried in dehumidifying dryers, transported with dry air, and blanketed with dry air or nitrogen in the extruder feed hopper. Other problems include contamination with foreign materials such as metal chips, screws, bolts, nuts, cardboard, rodents, and rodent droppings. Even so-called medical grade plastics are often contaminated. Of course, the contaminate will end up in the melt, so every precaution must be taken to avoid contamination by closing bags, covering hoppers, keeping a clean area around the extruders, and similar measures.
Some of the problems that can occur during extrusion are traceable to the processed raw material; that is, the raw material is not up to specification.
We will choose the proper plastic additives such as Colorants or UV inhibitors in different forms added to plastic extrusion manufacturing.
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